Pertussis toxin inhibits angiotensin II-mediated phosphatidylinositol breakdown and ADP-ribosylates a 40 Kd protein in cultured smooth muscle cells

G-Protein beta(3) subunit gene variant and left ventricular hypertrophy in essential hypertension

A functional genetic variant consisting of a C825T substitution in the GNB3 gene, encoding for the G-protein beta(3) subunit, has been associated with enhanced G-protein activation and cell growth. The aim of the study was to investigate the association of this polymorphism with left ventricular hypertrophy (LVH) in a sample of patients with essential hypertension. Left ventricular mass was assessed by 2-mode echocardiography in 86 patients with essential hypertension, and GNB3 C825T genotype was determined by polymerase chain reaction and restriction digestion. Thirty-seven (0.43) patients were homozygous for the C allele (CC), 40 (0.47) were heterozygous (CT), and 9 (0.10) were homozygous for the T allele (TT). The genotype distribution among the patients was in Hardy-Weinberg equilibrium. Values of left ventricular end-diastolic diameter (52.0+/-0.7 versus 48.9+/-0.9 mm, P=0.007), posterior wall thickness (11.3+/-0.2 versus 10.6+/-0.2 mm, P=0.042), and left ventricular mass index (152.7+/-4.4 versus 135.2+/-6.4 g/m(2), P=0. 023) were significantly higher in patients with CT and TT genotypes considered together (CT+TT) than in CC patients. The distribution of the genotypes was significantly different when comparing patients with LVH: 20 (0.33) CC and 40 (0.67) CT+TT patients had this complication, and 17 (0.65) CC and 9 (0.35) CT+TT patients did not (P<0.01). The frequency of the T allele was significantly different among patients with (0.40) and without (0.20) LVH (P<0.01). A logistic regression analysis showed that the association between the T allele and LVH was independent of age, mean blood pressure, body mass index, and alcohol consumption. The relative risk of LVH in patients bearing the T allele (CT+TT group) compared with CC hypertensive patients was 3.03 (95% CI 1.14 to 8.05). The findings suggest an association between LVH and the 825T allele in hypertensive patients.

Association between a polymorphism in the G protein beta3 subunit gene and lower renin and elevated diastolic blood pressure levels

Gi proteins mediate the intracellular effects of many vasoactive and proliferative stimuli. Recently such signaling was found to be enhanced in cultured cells of some hypertensive subjects. A polymorphism at position 825 (C-->T) of the G protein beta3 subunit gene (GNB3) was strictly related to this phenotype. The aim of the present investigation was to test the association between this polymorphism and blood pressure and plasma renin levels in humans. A population-based sample (n=608) was analyzed by questionnaire and characterized for blood pressure; plasma renin, prorenin, and aldosterone levels; and Gbeta3 C825T allele status. In individuals without antihypertensive medication (n=474; age range, 52 to 67 years), the polymorphism was mildly associated with diastolic blood pressure (CC: 88.6+/-0.3 mm Hg, n=218; versus CT: 90.1+/-0.7 mm Hg, n=209; versus TT: 91.8+/-1.7 mm Hg, n=47; P=0.02 for trend) but not with systolic blood pressure. Furthermore, the 825T allele was also significantly associated with lower renin and prorenin levels, whereas the aldosterone to renin ratio was elevated in these subjects. Significant associations between the 825T allele and diastolic blood pressure, plasma renin, and prorenin levels (inverse), and the aldosterone to renin ratio persisted after adjustment for age, gender, body mass index, and systolic blood pressure. Finally, when the entire sample was considered and an adjustment was made for covariates, the presence of arterial hypertension and the use of antihypertensive medication were both 1. 8-fold higher in the TT than in the CC genotype group (P<0.05 and P=0.06, respectively). This observation, if replicated in further studies, suggests a molecular mechanism that unifies a higher diastolic blood pressure, a lower renin level, and an elevated aldosterone to renin ratio, ie, a combination of features frequently found in patients with arterial hypertension.

Measurement of changes in sarcoplasmic reticulum [Ca2+] in rat tail artery with targeted apoaequorin delivered by an adenoviral vector

The physiologic relevance of Ca2+ release from the sarcoplasmic reticulum in arterial smooth muscle contraction is controversial. Therefore, we sought to measure changes in sarcoplasmic reticulum free [Ca2+] (i.e. [Ca2+]sr) in the intact rat tail artery. We exploited a novel technique to measure [Ca2+]sr with genetically targeted apoaequorin acting as a pseudo-luciferase rather than as classic aequorin. Intact rat tail arteries were infected with a replication deficient adenoviral vector (RAdER) containing the apoaequorin gene targeted to the sarcoplasmic reticulum. Addition of apoaequorin's substrate, coelenterazine, to the perfusate increased light production in a [Ca2+] dependent manner, consistent with apoaequorin action on coelenterazine. Within the limits of the photon counting system, imaging of infected rat tail artery segments revealed light production from the whole thickness of the vascular wall. Phenylephrine stimulation decreased apoaequorin generated light and induced a contraction. Washout of phenylephrine relaxed the tissues and increased light indicating refilling of the sarcoplasmic reticulum with Ca2+. Incubation in 10 microM cyclopiazonic acid, a SERCA inhibitor, did not alter apoaequorin generated light or induce a contraction. In the presence of cyclopiazonic acid, phenylephrine contractions were enhanced and apoaequorin generated light decreased further than that observed in the absence of cyclopiazonic acid. Cyclopiazonic acid also prevented the increase in apoaequorin generated light upon washout of phenylephrine, consistent with its inhibition of sarcoplasmic reticulum refilling. These results suggest that light production from targeted apoaequorin, delivered by a replication deficient adenovirus, is a valid measure of changes in [Ca2+]sr in the intact arterial wall. There appeared to be a correlation between Ca2+ release and contraction in these lightly loaded arteries.

Vasopressin induces arachidonic acid release through pertussis toxin-sensitive GTP-binding protein in aortic smooth muscle cells: independence from phosphoinositide hydrolysis

We previously reported that pertussis toxin (PTX) had little effect on arginine vasopressin-induced formation of inositol trisphosphate (IP3) in rat aortic smooth muscle cells [Kondo et al.: Biochemical and Biophysical Research Communications 161:677-682, 1989]. In the present study, we investigated the mechanism of vasopressin-induced arachidonic acid release in rat aortic smooth muscle cells. Vasopressin stimulated both the release of arachidonic acid and the formation of IP3 dose dependently in the range between 10 pM and 1 microM. The effect of vasopressin on arachidonic acid release was more potent than that on the formation of IP3. Quinacrine, a phospholipase A2 inhibitor, significantly suppressed the vasopressin-induced arachidonic acid release but had little effect on the formation of inositol phosphates. NaF, a GTP-binding protein activator, mimicked vasopressin by stimulating the arachidonic acid release. The arachidonic acid release stimulated by a combination of vasopressin and NaF was not additive. PTX partially but significantly suppressed the vasopressin-induced arachidonic acid release. In the cell membranes, PTX catalyzed ADP-ribosylation of a protein with an M(r) of about 40,000. Pretreatment of membranes with 0.1 microM vasopressin in the presence of 2.5 mM MgCl2 and 100 microM GTP markedly attenuated this PTX-catalyzed ADP-ribosylation of the protein in a time-dependent manner. These results strongly suggest that PTX-sensitive GTP-binding protein is involved in the coupling of vasopressin receptor to phospholipase A2 in primary cultured rat aortic smooth muscle cells.

Cholera toxin but not pertussis toxin inhibits angiotensin II-enhanced contractions in the rat portal vein

Angiotensin II (Ang II)-enhanced phasic contractions in the rat portal vein were concentration dependently inhibited by cholera toxin (0.1-10 micrograms/ml) and dibutyryl cyclic AMP (0.1-1 mM), but not by pertussis toxin (1 micrograms/ml), which suggests that Gi is not involved in the Ang II signal transduction pathway. It also seems likely that the effect of cholera toxin is due to its ability to increase cyclic AMP production through Gs.

Regulation of contraction and relaxation in arterial smooth muscle

Intracellular calcium concentration ([Ca2+]i)-dependent activation of myosin light chain kinase and its phosphorylation of the 20-kd light chain of myosin is generally considered the primary mechanism responsible for regulation of contractile force in arterial smooth muscle. However, recent data suggest that the relation between [Ca2+]i and myosin light chain phosphorylation is variable and depends on the form of stimulation. The dependence of myosin phosphorylation on [Ca2+]i has been termed the "[Ca2+]i sensitivity of phosphorylation." The [Ca2+]i sensitivity of phosphorylation is "high" when relatively small increases in [Ca2+]i induce a large increase in myosin phosphorylation. Conversely, the [Ca2+]i sensitivity of phosphorylation is "low" when relatively large increases in [Ca2+]i are required to induce a small increase in myosin phosphorylation. There are two proposed mechanisms for changes in the [Ca2+]i sensitivity of phosphorylation: Ca(2+)-dependent decreases in the [Ca2+]i sensitivity of phosphorylation induced by phosphorylation of myosin light chain kinase by Ca(2+)-calmodulin protein kinase II and agonist-dependent increases in the [Ca2+]i sensitivity of phosphorylation by inhibition of a myosin light chain phosphatase. I will review the proposed mechanisms responsible for the regulation of [Ca2+]i and the [Ca2+]i sensitivity of phosphorylation in arterial smooth muscle.

Angiotensin II-induced changes in guanine nucleotide binding and regulatory proteins

Systemic infusion of angiotensin II, a potent agonist, using doses that are initially subpressor, eventually produces sustained blood pressure elevation and reductions in glomerular capillary ultrafiltration coefficient characterized by enhanced signal transduction to angiotensin II and other agonists. In this setting, there is a significant increased affinity of angiotensin II binding to smooth muscle and glomerular mesangial receptors and enhanced sensitivity and magnitude of angiotensin II-induced decrements in cyclic AMP. Since G proteins are important modulators of binding and signal transduction, the present studies were designed to test the hypothesis that differences in the relative amounts of G proteins may be present and have accounted for differences observed. G proteins were identified and quantitated by isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis, radiolabeling in the presence of activated toxins with [gamma-32P]NAD+, immunoprecipitation, and immunoblotting. A 168% and 465% increase in pertussis toxin-catalyzed ADP ribosylation of alpha 40-41 was found in angiotensin II-treated groups over control groups for glomerular and mesenteric membranes, respectively. Immunoblotting revealed a 250% and 35% increase in the levels of the Gi isoforms alpha i-2 and alpha i-3, respectively, and a decrease of 53% in alpha i-1 from the angiotensin II-treated group. No differences were observed in cholera toxin labeling or immunoblotting of Gs. These results demonstrate multiple mechanisms whereby angiotensin-induced signal transduction can be modulated involving both the receptors and G proteins. These observed differences in G proteins in systemic and renal vasculature accompanying angiotensin II infusion suggest the possibility of a regulatory role in the pathophysiology of angiotensin II-induced hypertension and renal disease.

Pertussis toxin attenuates angiotensin II but not beta-adrenoceptor facilitation of noradrenaline release from rat kidney cortex

1. Angiotensin II (AII; 0.01 and 0.1 mumols/L), angiotensin I (AI, 0.1 mumols/L) and the beta-adrenoceptor agonist isoprenaline (0.1 mumols/L) all facilitated the stimulation-induced outflow of radioactivity from slices of rat kidney cortex incubated in [3H]-noradrenaline. 2. Treatment of rats with pertussis toxin (25 and 50 micrograms/kg i.v.) to inactivate G-proteins attenuated the facilitation caused by AII and AI, but not that caused by isoprenaline. 3. The hypothesis that isoprenaline enhances noradrenaline release by generating AII to activate facilitatory prejunctional AII receptors is not supported by the present study. The hypothesis predicts that pertussis toxin, by inactivating the G-proteins associated with AII receptors, should have inhibited the facilitatory effect of isoprenaline. This did not occur.

Prejunctional beta-adrenoceptors, angiotensin II and neuropeptide Y receptors on sympathetic nerves in mouse atria are linked to N-ethylmaleimide-susceptible G-proteins

We used the alkylating agent N-ethylmaleimide in order to investigate G-proteins linked to release-modulating prejunctional receptors of sympathetic nerves in mouse atria incubated with [3H]-noradrenaline. The receptors tested were facilitatory beta-adrenoceptors and angiotensin II receptors and inhibitory neuropeptide Y receptors. In order to evaluate the specificity of the N-ethylmaleimide treatment, we tested N-ethylmaleimide against the second messenger pathways that are linked to beta-adrenoceptors (adenylate cyclase) and angiotensin II (protein kinase C). The results show that a 60-min preincubation with N-ethylmaleimide (3 microM) abolished the facilitatory effect of isoprenaline (0.1 microM) and angiotensin II (0.1 microM) on the stimulation-induced release of noradrenaline and reduced the inhibitory action of neuropeptide Y (0.3 microM). N-ethylmaleimide had no effect on the stimulatory action of either phorbol dibutyrate (0.01, 0.1 microM), forskolin (10 microM), or a combination of 8-bromo adenosine-3'5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). However, at a higher concentration (10 microM), N-ethylmaleimide reduced the facilitatory effect of phorbol dibutyrate (0.1 microM) and the combination of 8-bromo adenosine-3',5'-monophosphate (90 microM) and 3-isobutyl-1-methylxanthine (100 microM). This suggests that N-ethylmaleimide at 3 microM but not 10 microM was selective for receptor-mediated modulation of noradrenaline release without directly affecting the adenylate cyclase (forskolin, 8-bromo adenosine-3',5'-monophosphate + 3-isobutyl-1-methylxanthine) or protein kinase C (phorbol dibutyrate) transduction pathways. In atria from mice pretreated with pertussis toxin (1.5 micrograms/mouse), N-ethylmaleimide preincubation (1 and 3 microM) resulted in a more pronounced reduction of the inhibitory action of neuropeptide Y (0.3 microM). The nature of this interaction is unclear. Since N-ethylmaleimide has been shown in other studies to inactivate G-proteins, the inhibitory effect of N-ethylmaleimide on prejunctional beta-adrenoceptors, angiotensin II receptors and neuropeptide Y receptors of sympathetic nerves may suggest that G-proteins are involved with these receptors, although other effects of N-ethylmaleimide on the receptor coupling processes cannot be ruled out. Moreover, it appears that the concentration of N-ethylmaleimide used is critical since a higher concentration (10 microM) resulted in non-specific effects on signal transduction mechanisms in the present experimental conditions.

The relationship between homotropic and heterotropic cooperativity for angiotensin receptors in smooth muscle

1. Angiotensin-induced contraction of smooth muscle is accompanied by both homotropic (receptor-receptor) and heterotropic (receptor-G protein) cooperativity. 2. Binding constants for angiotensins II and III at uterine smooth muscle receptors have been compared in bioassays and binding assays, using the competitive antagonist Sarmesin to verify the binding assay/bioassay interrelationship. 3. Agonist affinities determined from binding studies in the presence of GTP/S were found to be similar to the affinities observed in responding rat uterine tissues under conditions which eliminate positive homotropic cooperativity, suggesting that heterotropic cooperativity and homotropic cooperativity are interdependent events for smooth muscle contraction. 4. The data are consistent with an allosteric or autosteric mechanism of receptor function involving cooperativity between two agonist binding sites on the receptor. 5. The model has been used to calculate homotropic efficacies for angiotensins II and III from bioassay data and binding data, respectively.

Effect of phorbol ester and pertussis toxin on the enhancement of noradrenaline release by angiotensin II in mouse atria

1. Mouse atria were incubated with [3H]-noradrenaline, and the outflow of radioactivity due to electrical field stimulation (5 Hz, 60 s) was used as an index of noradrenaline release. Angiotensin II (0.01 and 0.1 microM) significantly enhanced the stimulation-induced (S-I) outflow of radioactivity. 2. Phorbol 12-myristate 13-acetate (0.001, 0.03, 0.1 and 1.0 microM), a protein kinase C activating phorbol ester, significantly enhanced the S-I outflow of radioactivity. When angiotensin II (0.1 microM) was present with the concentration of phorbol 12-myristate 13-acetate that was maximally effective in increasing the S-I outflow (0.1 microM), the enhancement of S-I outflow produced by angiotensin II was maintained. 3. Polymyxin B (70 microM), an inhibitor of protein kinase C, significantly inhibited the S-I outflow. Polymyxin B also inhibited the enhancement of the S-I outflow produced by angiotensin II (0.1 microM). 4. In another series of experiments mice were injected with pertussis toxin (1.5 micrograms per mouse), 4 days before their atria were removed. The effectiveness of pertussis toxin pretreatment was determined indirectly using carbachol. Carbachol caused a concentration-dependent fall in both the rate and force of beating of isolated spontaneously beating atria from mice pretreated with vehicle. This effect of carbachol was not seen with atria from mice pretreated with pertussis toxin. 5. Pertussis toxin pretreatment did not alter the enhancement of the S-I outflow of radioactivity produced by angiotensin II (0.01 and 0.1 microM). 6. These results suggest that angiotensin II receptor modulation of noradrenaline release is not mediated through either a pertussis toxin sensitive guanine nucleotide-binding protein or activation of protein kinase C.